Distribution system nowadays are usually equipped with protection devices with remote communication capabilities such as circuit breaker (CB) and line devices that are capable of providing sectionalizing, switching, reclosing features or any combination of these capabilities. Most of the CBs inside the substations have been equipped with electro-mechanical relays, which is a cost effective solution that can trip and reclose based on pre-configured intervals and times to de-energize a distribution power line in the event of possible line failure. In recent years, more advanced relay packages are installed in replacement of the electro-mechanical relays but the percentage of the older style relay is still very high in common distribution system due to cost. On another front, with increasing amount of distribution automation and protection equipment added to the distribution circuits, the complexity of the optimized control of such equipment in responding to an outage is calling for automatic switching schemes that can cut down switching time and increase efficiency. Generally, such automated switching schemes can be deployed based on utility preference using one of three philosophies:
The first one is “Centralized Control” whereby; all the inputs, solution algorithms, and control actions are handled by a “central” control system. An interface is created to allow users, in a control center, to interact with the software;
The second approach is “De-centralized Control”; whereby the inputs, solution scripts, and control actions are handled locally by a controller located in a distribution substation or other convenient location. An interface is created to allow users, in a control center, to interact with the software;
The last approach is “Hybrid Control”; whereby a combination of the above philosophies is deployed to restore different sections of a circuit, usually being upstream (restore from the normal source) and downstream (restore from a neighboring circuit).
The requirements from vendors that provide any of these systems are quite clear and standard: Valid system modeling including current status of all devices is needed. Valid fault sensing inputs, typically requiring a fault indication at each device to be considered in the scheme; Controllability of all devices in the scheme, which requires robust communication infrastructure; Failure sensing mechanisms, which are necessary to such system to tell if fault detection or switching device should be rendered ineligible.
Given the hardware limitation of electro-mechanical relays where they do not possess valid fault indication, which is one of the key requirement for enabling such automatic switching systems, Distribution Utilities seeking to deploy one of these switching systems are finding themselves in a quandary between not deploying the automatic restoration system on some circuits due to lack of qualified input information or spending capitol dollars to upgrade older devices (feeder breakers or line reclosers) or install standalone fault sensing equipment, and commonly having to go through a very lengthy process for such upgrades.